Grating couplers are commonly used in integrated optics for coupling light between integrated on-chip photonic waveguide structures and optical fibers. The grating couplers can couple light to and from the chip at any location, but the bandwidth can be limited due to the dispersive operating principle of the grating couplers.
Grating couplers can be formed in many different ways. For example, grating couplers can having a single-layer SiN top cladding (for products) or without the SiN cladding (used for wafer-level testing). Grating couplers with nitride top cladding suffer from limited coupling efficiency due to the additional reflection induced by the nitride cladding. The non-optimized top SiN top cladding will dramatically reduce the overall coupling efficiency of the photonic device.